"Poking through Eric's amazing treasure trove, I came across an AAR car code I'd never heard of, and that isn't listed in my 1/1943 ORER: XV. What is this?"
"XV" -- Box Car, ventilated. Similar to ordinary box car, only having ventilation, and suitable for transportation of produce or other foodstuffs not needing ventilation.
Car classifications were originally formulated by the railroad accounting associations and were adopted as "recommended" practice by the Master Car Builders Association in 1910. The "XV" classification existed simultaneously with "VA" and "VS" until 1929 when the American Railway Association omitted "XV". Along with omitting classification "XV", the "VA" and "VS" descriptions were expanded, and a new classification, "VM", was added.
The changes were all submitted by ballot to the ARA membership with all three receiving approval. All the changes took effect on March 1, 1930. Check your ORER for the classifications and descriptions...if you would like copies of the original classifications and their descriptions, as well as the 1929 changes, contact me and I will send you the information.